1. Field of the Invention
The present invention generally relates to ink jet printhead apparatus and, more particularly, to a drop-on-demand type ink jet printhead capable of producing gray scale images using either pulse width or voltage modulation to regulate the size of spots formed by droplets ejected from ink-carrying channels of the ink jet printhead
2. Description of Related Art
Ink jet printing devices use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets of ink, such that an ejected droplet may be precisely directed to a location specified by digitally stored image data for deposition thereat. Most ink jet printing devices commercially available may be generally classified as either a "continuous jet" type ink jet printing device where droplets are continuously ejected from the printhead and either directed to or away from a substrate, for example, a sheet of paper, depending on the desired image to be produced or as a "drop-on-demand" type ink jet printing device where droplets are ejected from the printhead in response to a specific command related to the image to be produced and all such ejected droplets are directed to the substrate for deposition.
Many drop-on-demand type ink jet printheads utilize electromechanically induced pressure waves to produce the desired droplets of ink. In one representative configuration thereof, a drop-on-demand type ink jet printhead has a horizontally spaced parallel array of internal ink-carrying channels. These internal channels are covered at their front ends by a plate member through which a spaced series of small ink discharge orifices are formed. Each channel opens outwardly through a different one of the spaced orifices. Within such a printhead, a volumetric change in fluid contained in the internal channels is induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to force a small, fixed quantity of ink, in droplet form, outwardly through the discharge orifice at a fixed velocity. The droplet strikes the paper at a specified location related to the image being produced and forms an ink "spot" having a diameter directly related to the volume of the ejected droplet.
Due to their ability to produce a spot at any location on a sheet of paper, ink jet and other non-impact printers have long been contemplated as being particularly well suited to the production of continuous and half tone images. However, the ability of ink jet printers to produce continuous and half tone images has been quite limited due to the fact that most ink jet printheads can only produce droplets having both a fixed volume and a fixed velocity. As a result, ink spots produced by such droplets striking a sheet of paper are of a fixed size, typically in the range of 120 .mu.m to 150 .mu.m, and the same intensity. Additionally, all ink jet printheads use a fixed resolution, typically 300-400 dpi (or "dots per inch") or lower, to place droplets on a sheet of paper. In contrast, a typical high quality half tone image produced using offset printing techniques uses variable sized spots at resolutions of up to 240 dots per inch.
Due to the aforementioned limitations, ink jet printheads have heretofore utilized spot density, as opposed to spot size, when attempting to produce a gray scale image. To do so, the ink jet printhead creates various shades of gray by varying the density of the fixed size ink spots. Darker shades are created by increasing spot density and lighter shades are created by reducing spot density. Producing a gray scale image in this manner, however, reduces the spacial resolution of the printer, thereby limiting its ability to produce finely detailed images. Furthermore, the more levels added to the gray scale, the greater the resultant degradation of the printer's spacial resolution. A second proposed solution has been to direct multiple droplets at a single location on the sheet of paper to form variably sized spots. While such a method can produce the variably sized spots necessary to produce a gray scale image, such a technique tends to reduce the operating speed of the printer to an unacceptably low level. Furthermore, this technique may also produce elongated or elliptical dot patterns.
It can be readily seen from the foregoing that it would be desirable to provide an improved drop-on-demand type ink jet printhead configured such that the size of ink spots produced thereby is readily modulatable to produce a gray scale. It is, therefore, an object of the present invention to provide such an improved drop-on-demand type ink jet printhead.